tenases

Tenases are enzyme complexes in the coagulation cascade that catalyze the activation of factor X to factor Xa on a phospholipid surface in the presence of calcium ions. There are two principal tenase complexes: intrinsic tenase and extrinsic tenase.

Intrinsic tenase consists of factor IXa as the catalytic protease and factor VIIIa as a cofactor, assembled on a negatively charged phospholipid surface—typically on activated platelets—in the presence of Ca2+. Factor IX is activated to IXa by factor XIa, and factor VIII is activated to VIIIa by thrombin. The IXa–VIIIa complex cleaves factor X to Xa, which then participates in the common pathway by forming the prothrombinase complex with factor Va to convert prothrombin into thrombin.

Extrinsic tenase comprises tissue factor (TF) bound to factor VIIa. The TF–VIIa complex activates factor X to Xa (and also IX to IXa) on a phospholipid surface in the presence of Ca2+. Tissue factor is exposed at sites of vascular injury, and factor VII is activated to VIIa by inflammatory signals or injury. This complex initiates thrombin generation when the intrinsic pathway components may be limited.

Common features of both tenases include dependence on calcium ions and assembly on cell membranes or phospholipid surfaces, facilitating localized thrombin generation. Clinically, deficiencies or inhibitors of the involved factors affect tenase activity: hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency) impair intrinsic tenase; factor VII deficiencies affect extrinsic tenase.

The term “tenase” thus denotes these enzyme complexes that activate factor X, forming a key control point in thrombin generation and coagulation.